Investigation of the Gate Bias Stress Instability in ZnO Thin Film Transistors by Low-Frequency Noise Analysis

To investigate the electrical instability mechanism under the application of gate bias stress and relaxation, the $1/f$ noise spectra of two different ZnO thin-film transistors (TFTs) were analyzed. In terms of gate bias dependence ($S_{\text{IDS}}/I_{\text{DS}}$), both devices followed a mobility fluctuation model based on the traps in their channel layers prior to and after stress. Device A (channel thickness: 20 nm), recovered its initial noise parameter ($\alpha_{\text{app}}$) after relaxation, in exact agreement with the current--voltage ($I$--$V$) measurement results; this shows that in device A, the dominant phenomenon under the application of stress was temporary charge trapping at grain boundary traps. However, in device B (channel thickness: 80 nm), $\alpha_{\text{app}}$ did not recover its initial values after relaxation, and transfer parameters, such as $V_{\text{TH}}$, mobility, SS, and $N_{\text{t}}$, degraded after the gate bias stress. Moreover, after the stress, device B showed a reduced gate insulator breakdown voltage. The electrical degradation seen in device B can be explained by trap creation and/or charge injection near channel/gate oxide interfaces, including those within the channel layer.